1. Field of the Invention
The present invention relates to a secondary battery. More specifically, the present invention relates to a built-in charge circuit for a secondary battery and a secondary battery with the built-in charge circuit.
2. Description of the Related Art
With the rapid advance and development in recent years of electronic, information communication and computer industries, there is increasing use of portable electronic devices. Secondary (rechargeable) batteries are largely used as power sources for portable electronic devices.
Pack-type batteries are currently widely used as secondary batteries. A pack-type secondary battery generally has a structure in which a bare cell storing and supplying electrical energy and a protection circuit controlling charge/discharge of the bare cell are integrated into one unit.
The secondary battery utilized in a portable electronic device such as a portable computer is constructed such that the battery is charged in a state where it was mounted on the portable electronic device. Charge circuit built-in secondary batteries have also recently been used which are configured such that the battery can be charged even in a state where it is separated from the portable electronic device. Unfortunately, conventional charge circuits built in the secondary batteries are potentially susceptible to damage due to an electric current arising from the reversed charge connection of circuit components.
FIG. 1 shows a block diagram of a secondary battery having a conventional charge circuit built therein. Referring to FIG. 1, a secondary battery 10 includes a bare cell 20, a protection circuit section 30, and a charge circuit section 40. The charge circuit section 40 is formed in such a manner that the secondary battery 10 can be charged in a state where it is detached from a portable electronic device (not shown). The charge circuit section 40 includes a first switching element 42 connected to a positive electrode charge terminal 41 and a reverse-current prevention element 44 connected to a negative electrode charge terminal 43, such that the charge circuit section 40 is operated in a step-down fashion where an input voltage is higher than an output voltage. A duty ratio of the first switching element 42 is controlled by a pulse width modulation (PWM) controller 45, so a desired charge voltage is obtained. A given charge voltage obtained by means of the first switching element 42 is stored in a choke coil 46 and is applied to the protection circuit section 30. The reverse-current prevention element 44 prevents a flow of the charging current to the negative electrode charge terminal 43. When the first switching element 42 is turned off, a field effect transistor (FET) 48 of the reverse-current prevention element 44 is turned on, so the energy stored in the choke coil 46 exits through a forward path of a diode 47 of the reverse-current prevention element 44.
The secondary battery 10 can be charged even in a state where it is mounted on the portable electronic device. This is carried out through a positive electrode charge/discharge terminal 31 and a negative electrode charge/discharge terminal 32, which are connected to the protection circuit section 30. However, when the positive electrode charge/discharge terminal 31 and the negative electrode charge/discharge terminal 32 are wrongly reverse-connected to the portable electronic device, this leads to the occurrence of reversed charge. The protection circuit section 30 detects the reversed charge event to thereby turn off a charge switching element 33 and a discharge switching element 34, resulting in protection of the protection circuit section 30. However, an undesirable current (I), as indicated by a dotted line, is generated in the charge circuit section 40. This current (I) will sequentially pass through the negative electrode charge terminal 43, the reverse-current prevention element 44, the choke coil 46, and the positive electrode charge/discharge terminal 31. A flow of the current (I) leads to heat generation of the reverse-current prevention element 44, which will consequently contribute to damage of the reverse-current prevention element 44.